Optical retinal imaging is class of a medical imaging techniques that is applied in medical diagnosis and treatment methods to examine the fundus of an eye. A number of these methods is based on the scanning of a beam of imaging radiation across the fundus. The scanning is performed step-by-step, and image data for a pixel (or for a line of pixels) is obtained from reflected radiation during each scanning step. These optical retinal imaging techniques for example include optical coherence tomography (OCT), confocal scanning laser ophthalmology (cSLO), and line scanning laser ophthalmology (lSLO).
Optical coherence tomography (OCT), for example, is a medical imaging technique that is based on interferometry using a reflected portion of light penetrating into biological tissue. OCT is for example applied in ophthalmology to obtain detailed images of sub-surficial and surficial structures on and below the retina of an eye. In this field, OCT has become a well established method of diagnosing many eye diseases.
In OCT, a beam of imaging radiation is impinged on for example the fundus of an eye and the reflected part is led through an interferometer wherein it interferes with a reference beam. By scanning the imaging radiation across an area of the fundus, a (three dimensional) image may be obtained from the interference pattern. Although OCT has the potential to enable ophthalmologic imaging at high resolution, at present the lateral resolution is strongly limited by eye motions of the eye under examination. These motions may be divided in three different types, including eye tremor, drift and micro saccades. The first one—tremor—may be characterized as a vibrating motion of the eye having a typical frequency around 90 Hz and a typical amplitude of approximately 1 μm. The second type of motion may cause the eye to move across a distance of for example 0.1 mm but at a typical frequency of 1 Hz. The micro saccades may also have a typical period of 1 Hz, with a duration of only 25 ms typically and an amplitude of 25-100 μm. The peak velocity of these micro saccades may be 10 mm/sec.
Adaptive Optics OCT systems presently under development, will enable to reduce the ocular resolution of the system down to approximately 3 μm (micrometer). To enable measurement at this resolution, residual eye motion must somehow be controlled or compensated such that the error introduced is typically below 1 μm (micrometer).
To improve accuracy, OCT systems and other retinal imaging systems for ophthalmologic purposes sometimes cooperate with tracking devices or tracker camera's. The tracker camera may for example include a fundus camera which generates a reference image from which any eye movements can be detected and analyzed. Additional correction optics may then correct the imaging beam to impinge on the correct location on the fundus. The taking of the reference image and the correcting of the beam is usually performed after each scanning step correct for any eye motion occurring during scanning. However, the present tracking devices are still not accurate enough to compensate for eye motion sufficiently, to obtain the required accuracy for correcting the optical retinal imaging system.